There are known in the art a variety of outdoor signs for presenting to the viewer large display numerals, letters, or text, one common version being an outdoor sign that portrays the time of day, the time being updated on a minute by minute basis. One such type of sign, characterized by a substantial economy of power as contrasted with self luminous displays, is disclosed in U.S. Pat. No. 4,015,255 issued to Wood March 29, 1977, wherein each character to be displayed is formed from a matrix of closely spaced discs rotatable about an axis normal to the viewing axis. One side of each disc is lightly colored, the other side being dark. Each disc is selectively actuated by a momentarily energized electromagnetic actuator to present to the observer on command either the bright side of the associated disc or its dark side. By well-known means, selected groups of discs are driven to present their appropriate faces outward so as to present, when seen from a distance, a desired number or letter.
Because of the nature of the electrical control sequence that is necessary to actuate such arrays from one configuration to the next, particularly where rapidly changing displays are involved, it is desirable that each individual indicating element be rotated quickly and positively from one of its two positions to the other, and with a minimum of energy expenditure. Until all elements of a given configuration have been actuated to their desired new positions, they cannot be subsequently re-actuated to form a new character. As will be discussed below, the requirement for rapid positive actuation has hitherto resulted in excessive power demand by the electromagnetic actuators. This problem is intimately related to the poor arresting and rebound properties of the rotation-terminating limit stops hitherto known in the art, as will be explained.
Displays of the type mentioned in the above cited patent have their individual elements actuated by mounting a permanent magnet on the rotating display element, and have an associated solenoid mounted on an adjacent stationary part of the sign structure. The solenoid is energized in one direction or the other to rotate the magnet and its associated indicating device in the desired direction.
A problem arises in securing an accurate, reliable bounce-free termination of element rotation to avoid sign-change flutter, while at the same time holding energy expenditure to a minimum. In practice this has proven to be extremely difficult to obtain. Some form of limit stop must be employed to arrest the motion of the display element in both directions, and the use of conventional stops gives rise to flutter problems. A positive locking feature can be secured at the expense of additional power at a given sign-changing speed if a strongly magnetizable (high remanence) core is used in the solenoid, so that upon removal of the current excitation the core remains permanently magnetized to maintain a strong attractive force against the associated display element magnet, thereby maintaining this strong attractive force between successive energizations to prevent rebound.
As is well known to those knowledgeable in the electromagnetic art, each successive reversal of solenoid current to drive the element in the opposite direction must not only magnetize the core in the opposite direction, but also it must wipe out the previous residual induction (remanence) in the solenoid core. Thus, the electrical power demand of the system is increased to an undesirable degree. Alternatively, if a magnetically "soft" solenoid core is employed, a long duration holding current must be maintained in the solenoid throughout the possible rebound period, again adding to the power requirements of the system.
The applicants have experimented with a variety of resilient stop limiter elements to absorb the kinetic energy of the indicating display elements at the end of a rotation stroke, but prior to the present invention the applicants found none that satisfactorily eliminates rebound unless used in conjunction the previously mentioned energy-wasteful approaches. Moreover, any such resilient stop limiting element must undergo a very great number of impacts during its useful life without undergoing substantial deterioration. Thus, a wear-resistant design is needed in such elements in order to prolong their useful life.
It would be reasonable to assume that an adequate suppression of rebound could be achieved simply by causing the rotating element to engage a spring loaded member of intermediate mass, elementary momentum transfer from the element to the mass thus suppressing the rebound. Although theoretically attractive, the applicants have repeatedly attempted using various limit stop designs based upon such principles without success to achieve the necessary anti-rebound properties.
Thus, there has been a need for a shock absorbing limit stop for such display elements which has substantially improved wearing properties, while still allowing rebound-free termination of an impulsive actuation stroke initiated by momentary actuation of the actuating solenoid, without requiring a subsequent holding current or permanent magnetization to prevent rebound. To the applicant's knowledge, prior to the invention which is the subject matter of this disclosure, no satisfactory solution to these problems had been found.